Microtiter or multi-well plates are becoming increasingly popular in various chemical and biological assays. Further, high-density format plates, such as 384, 864 and 1536 well plates, are beginning to displace 96-well plates as the plate of choice. Many of the assays conducted in multiwell plates employ some type of light detection from the plate as the reporter for positive or negative assays results. Such assays include fluorescence assays, luminescence assays (e.g., luciferase-based assays), phosphorescence assays, scintillation assays, and the like. In particular, with the advent of solid phase scintillating materials, capsules and beads, homogeneous scintillation proximity assays (SPA) are now being performed with increasing frequency in multiwell plates.
Detection of light signals from multiwell plates in the past has typically been done using plate readers, which generally employ a photodetector, an array of such photodetectors, photomultiplier tubes or a photodiode array to quantify the amount of light emitted from different wells. Such plate readers have been disclosed, for example, by Russell, et al., U.S. patent Ser. No. 4,810,096, issued Mar. 7, 1989, and VanCauter, et al., U.S. patent Ser. No. 5,198,670, issued Mar. 30, 1993. Although plate readers can detect the total light from each well, they have a number of limitations. For example, plate readers are typically not capable of resolving discrete light sources in a single well, so they could not be used, for example, to differentiate light from different beads in one well. Further, most plate readers have fewer photodetectors than there are wells in the plate, so at least some wells must be read serially, adding to the time required to complete the assays. This becomes a substantial problem in assays where the light signal is so low that each well needs to be in the detection field for an extended period of time (e.g., tens of minutes). In addition, most currently-available plate readers have been designed for 96-well plates. Although some can be adapted for, e.g., 384-well plates, the adaptation does not result in any significant increase in throughput, since a 384-well plate going through a modified 96-well reader typically takes four times as long to read as a 96-well plate.
Another technique that has been applied to the detection of light from multiwell plates is imaging. Prior art imaging systems typically comprise a standard 50-55 mm f1.4 photographic lens coupled to a camera. While such systems can be used to image an entire multiwell plate, and theoretically provide resolution of discrete light points within individual wells, they have poor sensitivity, even when coupled to efficient cameras, so that many assays still require imaging times of tens of minutes or more. Other assays, such as SPA bead-based assays, cannot be performed at all due to lack of sensitivity. Further, images acquired with such systems suffer from vignetting and lateral distortion effects, making it difficult or impossible to compare signals from center portions of the plate with signals from lateral wells.
The present invention provides lenses and systems which overcome the above-described disadvantages of prior art methods of light detection in multiwell plates. In particular, the present invention provides, for the first time, a doubly telecentric lens-based system with the ability to image SPA bead-based assays. The telecentric lens of the invention is economical to manufacture due to a design employing a minimal total number of lens elements, the use of spherical lens elements, and generous tolerance limits in lens fabrication. Further, the telecentric lens of the present invention is the first such lens to provide an unprocessed image of a multiwell plate that is substantially free from vignetting, chromatic aberration and distortion.